9/10/2012. Computed Radiography Chapter 3 Physics and Technology. What is Computed Radiography?

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1 Computed Radiography Chapter 3 Physics and Technology This presentation is a professional collaboration of development time prepared by: Rex Christensen Terri Jurkiewicz and Diane Kawamura Today s Humor: More Medical Anomalies What is Computed Radiography? Or what is CR? It is xrays passing thru the patient being picked up by a digital detector. Photostimulable luminescence (PSL) is seen due to phosphors being excited by the xrays. Other names for CR are SPR storage phosphor radiography and DSP digital storage phosphor radiography 1

2 Computerized Radiography The image becomes an apparent or visible when the phosphors are read by scanning with a laser beam which produces light. This is converted into electrical signals History 1983 CR started being used for diagnostic radiography. Fuji started with FCR 101 followed by Kodak, and Agfa. Now there are four manufacturers with the addition of Konica. Imaging System Four Steps: 1. Image Acquistion 2. Image plate scanning and erasure 3. Image processing 4. Image display + implementation issues: Use of grids, radiation exposure, and technique selection. 2

3 Physics of CR Image Formation 1. The imaging plate has photostimulable phosphors. 2. To image the latent image a laser beam with a certain wavelength must scan the plate producing luminescence or PSL (photostimulable luminescence). 3. Imaging plate is then erased by high intensity light (white light) that gets rid of any residual image. IP is then ready to be used again. PSP The photostimulable phosphor on the imaging plate must have good absorption efficiency of xrays and must be stimulated by a heliumneon laser. It must also be compatible with the photomultiplier tube and luminescence must be shorter the 1 microsecond. Phosphors must be able to store the latent image for several hours without compromising the signal from the IP Phosphors Phosphors meeting the above criteria are: Barium Fluoro Halide:Europium Halide can be chlorine, bromine, or iodine or a mixture of them. The phosphor is usually doped with Eurpopium because it is an activator which increases the efficiency of the PSL. 3

4 Latent Image Formation Electrons move from a ground state to the higher energy state when the europium atoms are bombarded by x rays. The xray s travel to the FCenter (F comes from farbe meaning color) The number of trapped electrons are equal to the absorbed radiation. Latent Image Formation This is how the latent image is formed. The exposure of the plate causes it to fluoresce for a brief duration. To make the image visible the plate must be read in a reader/processor as it is scanned by a laser beam. This is photostimulated excitation. PHOTOSTIMULABLE LUMINESCENCE Compton and photoelectric xray interactions occur with outershell electrons Outershell electrons are sent into an excited, metastable state Xray interaction with a photostimulable phosphor results in excitation of electrons into a metastable state. 4

5 PHOTOSTIMULABLE LUMINESCENCE Over time, the metastable electrons return to the ground state on their own When metastable electrons return to their ground state, visible light is emitted. PHOTOSTIMULABLE LUMINESCENCE Exposing the phosphor to intense infrared light from a laser accelerates or stimulates the process of the electrons returning to the ground state (50%). PHOTOSTIMULABLE LUMINESCENCE The storage phosphor screen appears white because the small photostimulable phosphor particles scatter light excessively Turbid is the term used to describe the excessive scattering of light Photostimulable phosphor particles are 3 to 10 mm The photostimulable phosphor particles are randomly positioned throughout the binder 5

6 Reader/Processor The laser must be able to be absorbed by the FCenters. This is what causes the trapped electrons to move up to the conduction band allowing them to be free, and the Europium is allowed then to return to its ground state. This is when there is the emission of a bluishpurple light or the luminescence. Laser Light The lasers today are semiconductor lasers that produce light with a 680nm wavelength, compared to the older He Ne lasers that produce light at 633nm which was used in the earlier CR units. Computer Radiography (CR) plate is photostimulable phosphor radiation traps electrons in high energy states higher states form latent image X R a y P h o t o n P h o t o n p u m p s e l e c t r o n t o h i g h e r e n e r g y s t a t e H i g h e r E n e r g y E l e c t r o n S t a t e L o w e r E n e r g y E l e c t r o n S t a t e 6

7 Reading Imaging Plate reader scans plate with laser laser releases electrons trapped in high energy states electrons fall to low energy states electrons give up energy as visible light light intensity is measure of incident radiation Lower Energy Elect ron St at e Higher Energy Elect ron St at e Lower Energy Electron State Laser Beam Analog to Digital Image The light emission is then collected by a collection device and sent to the photomultiplier tube which then produces an electrical signal, it is digitized by the ADC and sent to the computer for processing. Fading This is the time it takes for a latent image to disappear. That is why it is important to have the image plate read as soon as possible after the exposure. The PSL decreases by 25% within 8 hours between exposure and reading. 7

8 CR Technology The Imaging plate The imaging reader The display Imaging Plate There are 2 types of plates standard resolution and high resolution The imaging plate is the detector that is used in CR imaging The plate consists of: A PSP layer on a base for support. Two protective layers Electroconductor layer Light shielding layer. 8

9 Film Screen vs. IP Layers The purpose of the electroconductive layer is to reduce any static electricity problems when the IP is transported into the reader. PSP Layers A high resolution PSP layer is thinner than the standard resolution layer, and has a sharper image due to less lateral spread of the laser light. The thicker phosphor layer IP s have faster speed than the high resolution IP s. High resolution IP s will be used for small parts to increase the resolution. 9

10 Imaging Plates Housing the imaging plate is a lightweight aluminum or aluminum honeycomb panel on the back which is designed to prevent backscatter radiation, and a radiolucent front. Imaging Cycle The imaging plate is reusable for hundreds of exposures. There are at least 3 cycles the plate goes through: 1. IP is exposed to xrays. 2. The imaging plate is read in the reader rendering a latent image visible. Fluoroescent Bulbs Erase the Plate 3. The IP plate is erased using high intensity light to remove residual energy. 10

11 Imaging Reader Types The reader or scanner makes the latent image visible. The electrical signal that is generated is amplified and digitized. A. Cassette based system and B. Cassetteless systems. Cassette Based Systems They use imaging plates of various sizes as in filmscreen systems. The IP must be taken to the reader to be scanned to acquire the image, and for the residual to be erased. There are several transport systems that can damage the plate. Cassetteless Systems They contain a single fixed imaging plate. There is no contact with the imaging plate in the unit as it is read. The image is acquired using a scanning technology which is appropriate to each system. 11

12 Scanning the IP There are two main types of scanning mechanisms: 1. Point Scan Reader 2. Line Scan Reader P. 55 Point Scan Reader A. Laser source B. Transport mechanism C.Light channeling guide D. Photodetector E. Analog to Digital converter IP is removed from cassette and placed on transport mechanism The IP is on the slow scan direction The laser is on the fast scan direction Point Scan cont d The emitted light from the laser scanning is filtered and collected by a light channeling guide or light collection optic. The light is sent to the photodetector, photomultiplier tube, or CCD. It then becomes an electrical signal which is amplified and digitized Digitization involves sampling and quantization of the analog signal. 12

13 Line Scan Readers A.There are several linear laser sources B. Shaping Lens system C. Linear array of CCD photodetectors The laser beam is collected and shaped by the lens system to scan the IP line by line. The PSL is collected by the CCD linear photodetector array. This is why it is faster than point scan readers. Dual Sided Readers Using two sets of photodetectors to capture the PSL from both front and back of the IP allows for less signal to noise ratio, improving the image quality. Also a thicker phosphor layer can increase absorption of the xrays improving efficiency 13

14 CR Readers The IP must be erased for subsequent exposures. This is done in the reader by exposing it to high intensity light that is brighter than the laser light, to get rid of any residual image on the IP. The imaging plate should be erased if not used for an extended period of time. Workstation Allows: Input of patient information Region of exposure Image preview Image processing Quality assurance Image printing Sending images to PACS Workstation Consists of : Processing computer Monitor (CRT or LCD) Keyboard Mouse Some may have barcode readers and magnetic card reader. 14

15 Digital Image Processing Picture is of colliding galaxies taken from Hubble telescope Digital Image Processing Brightness and contrast are controlled by the workstation. Spatial frequency filtering (high pass edge enhancement/low pass smoothing is also possible through the workstation Original High Pass Low Pass (sup low freq) (sup high freq) PreProcessing Operations Preprocessing is necessary to make corrections on the data received from the imaging plate. 1. Exposure field recognition: This pertains to the size of the field of view. That is why centering, and coning is so important. 15

16 Histogram 2. Histogram analysis the computer analyzes the exposure field and makes an analysis of what was imaged. So if it was centered incorrectly the image will be light or dark. Measured histogram is the actual histogram of the image taken Stored histogram is the predetermined histogram for the study (ie forearm predetermined average histogram. Histogram A histogram is a graph of the number of pixels in the entire image or part of the image having the same gray levels (density values) plotted as a function of the gray levels Grayscale Rendition 3. Grayscale rendition It is the determination of the minimum and maximum penetration and sets the luminance presentation according to those values obtained during the exposure. This is where the look up table comes into play. 16

17 Exposure Recognition It is the visual indicator for the technologist letting him/her know whether the exposure was appropriate. This can be used as a form of quality control to optimize the quality of the image and limit the exposure rate to the patient. Exposure Indicators Fuji Medical Sensitivity number (S number) Agfa Medical log of Median (lgm) KodakExposure Index (EI) They each use a different name for the same thing this is the exposure recognition indicator. Post Processing These enable the technologist or radiologist to manipulate the image to optimize it for viewing purposes. Increase/decrease contrastbrightness, sharpen the image, smooth the image, enlarge an area, lighten or darken the image, remove the bone, or soft tissue. All of this is possible with post processing. 17

18 Edge Enhancement Smoothing Contrast Manipulation W 226 W 100 W 500 = Postcontrast Image Precontrast Image Mask Subtraction Image 18

19 Digital Imaging Processing Contrast Enhancement Accomplished by windowing Window width encompasses the range of densities within an image A narrow window is comparable to the use of a highcontrast radiographic film Image contrast is increased A narrow window is valuable when subtle difference in subject density need to be better visualized Narrow window increases image noise and the densities outside of the narrow window are not visualized Digital Imaging Processing Contrast Enhancement Increasing the width of the window allows more of the gray scale to be visualized or more latitude in the densities of the image 19

20 Windowing Windowing is a digital image processing technique that also changes the contrast and brightness of an image. The illustration shows the range of pixel values (gray levels) and displayed image contrast range on a digital image. Windowing A digital image is made up of numbers. The range of number is the window width (WW) The center of the range is defined as the window level (WL). The WW controls image contrast. The WL controls the brightness. Windowing The displayed WW and WL values are always shown on the image. Narrow WW provides higher image contrast (shortscale contrast) and a wide WW will show an image with less contrast. 20

21 Windowing If the WL is increased, the image becomes darker since more of the lower numbers will be displayed. Exposures of Imaging Plates Underexposure leaves a noisy image Overexposure although often has a good quality image, over exposes the patient to unnecessary radiation. Hence, dose creep must be avoided at all costs. The saying when in doubt, black it out is prevalent in digital imaging, but should be minimized to prevent overexposure to the patient. Exposure Indicators It is a numerical value that is used to demonstrate the exposure. As previously state the different companies use different indicators. SNumber EI (exposure indicator) LgM (log of median values) 21

22 Sensitivity Number Fuji S=200 Exposure to the IP The exposure is inversely proportional to the exposure. High S number is a low exposure and a Low S number is a high exposure Exposure Index Kodak EI=Log(exposure in mr)x The EI is directly proportional to the exposure. A high EI will be a high exposure and a low EI will be a low exposure. 1mR will have an EI of 2000 LgM Values Agfa lgm=2.2+log (exposure in mr) 1mR will equal increase or decrease will double or half the exposure level. 22

23 EI Guidelines for Quality Control The challenge is to minimize the radiation exposure to the patient, while maintaining optimal exposure image quality. The exposure indicator is what allows the technologist to accomplish this. Spatial Resolution Size of the pixels help to determine the resolution of an image. The smaller the pixels the better the resolution. Resolution The number of pixels in a given area defines the resolution of an image One pixel x 1,000 pixels 23

24 Dynamic Range (bitdepth) 1 bit 8 bit grayscale 8 bit color 24 bit color (GIF) (GIF) (JPEG) presentations/2002bcla/17 1 bit = black or white 8 bits = 256 shades 16 bits = thousands 24 bits = millions 36 bits = billions The bit depth has an effect on the number of shades of gray, hence the density resolution of the image. Noise Electronic Noise (System): You cannot control the system noise, it is inherent in the machine. Quantum Mottle (noise): This is determined by the number of photons hitting the detector. When the number of photons is too low there is more noise, and the reverse high number of photons lower noise. 24

25 Digital Imaging Processing Modulation Transfer Function MTF is the spatial frequency response of an imaging system or a component; it is the contrast at a given spatial frequency relative to low frequencies. Spatial frequency is typically measured in cycles or line pairs per millimeter (lp/mm) High spatial frequencies correspond to fine image detail. The more extended the response, the finer the detail the sharper the image. Digital Imaging Processing MTF If you were to look at a wire under and electron microscope, you would see that the wire is not smooth, it is actually pitted or has divots on the outer surface. Information or data is transferred over the outer surface of a wire. These divots actually absorb bits of data as they pass by this degrades the image Most equipment runs at 99% meaning 1% of data is lost when being transferred. DQE Detective Quantum Efficiency The detector receives the exposure converting it to an image. The DQE is a measurement of the efficiency in which the detector performs this task. It includes the signal to noise ratio and the system noise in the value. You want no loss of information. Perfect detector is 1 or 100% 25

26 Noise Assume the dot is noise Artifacts Digital artifacts come from many sources. Hardware, software, and objects imaged. Many artifacts arise from the imaging plate it is handled frequently, and should be cleaned often. Incorrect storage of IP and use of grids. CR reader is another source of artifacts. Poor Screen Cleaning 26

27 CR IP in upside down Dirty Reader CQI Continous Quality Improvement was established by JCAHO (Joint Commision on Accreditation of Healthcare Organizations). It states that every employee plays a vital role in continuous quality improvement. 27

28 Quality Assurance This is the system that assures quality patient care. Administrative dealing mainly with assessment measures or outcome. Quality Control This is technical aspect dealing mainly with the quality and maintenance of equipment and dose to the patient. The point of CQI, QA and QC is to maintain and improve the quality of service, dose, images, and diagnosis of the patient all at the lowest cost possible. Test Tools for CR Inspect the Imaging Plate Test the monitor for resolution, contrast, brightness, linearity, collimation, etc. Wire mesh can be used for contact, antiscatter grid, densitometer for hard copy images, and phantoms 28

29 The End of Chapter 3 Picture of Earth from Hubble telescope Thanks to als.net/article11.html for some of the images obtained in this ppt. 29